The invention relates to a method for the electrochemical measurement of an analyte concentration in vivo by means of a fuel cell which catalytically converts the analyte to be measured with an enzyme contained in an enzyme layer and supplies an electrical voltage between an anode and a cathode.
As is known from U.S. Pat. No. 3,837,339, fuel cells are used as electrochemical sensors for in vivo measurement of analyte concentrations, for example, glucose concentration. Therein, the analyte to be measured is catalytically converted as the fuel of the fuel cell. The fuel cell described in U.S. Pat. No. 3,837,339 oxidizes glucose on the anode, thereby generating gluconic acid. Oxygen is reduced on the cathode.
In order to improve the efficiency of these reactions, it is known to provide the cathode with an enzyme layer which contains an enzyme for catalytically converting the analyte and to apply an enzyme layer containing an enzyme to the anode for the catalytic reduction of oxygen. For example, such fuel cells are known from U.S. Publication No. 2005/0118494. In these fuel cells, the anode is covered with an enzyme layer which contains glucose oxidase and the cathode is covered with an enzyme layer which contains laccase. In this manner, the anode reaction, i.e. the oxidation of glucose, and the cathode reaction, i.e. the reduction of oxygen, can be accelerated.
A problem arising in connection with the in vivo measurement of analyte concentrations with such sensors is that the intensity of the measuring signal can be affected by an oxygen deficiency in the environment of the cathode. In particular in the event of prolonged operation, depletion of the oxygen concentration can occur in the environment of the cathode, the oxygen concentration being already subject to great variations in subcutaneous fatty tissue. This may falsify the measuring signal. A further problem is that intermediate products that are harmful to health or even toxic, for example, hydrogen peroxide, can often develop in the catalytic conversion of analytes. If such intermediate products exit from the sensor, this may result in inflammation and require premature removal of the sensor.